When you want to render a template to string in controller you have two options: using ERB methods explicitly or (considering you’re in Rails) render_to_string.
If’re to use ERB, you should first get string representation of your template, then pass it to the ERB and get result for current binding. The code:
raw_template = File.read('path/to/your/template.html.erb')
erb_template = ERB.new(raw_template)
erb_template.result(binding)
This method is universal and may be used anywhere in Ruby.
However, since you’re in Rails this approach may look too complicated as ActionController provides a handy helper: render_to_string. Just give it the view name and you’re done. Consider layout: false option in case you don’t want your default layout to be rendered with the template:
render_to_string('your/template')
render_to_string('your/template', layout: false)